Demography of three dominant sedges under contrasting grazing regimes in the High Arctic

摘要

Tiller demography of Carex aquatilis ssp. stans, Carex membranacea, and Eriophorum angustifolium ssp. triste was investigated in ungrazed and grazed high arctic vegetation on central Ellesmere Island. Canada. Tiller birth, growth, flowering and death were studied from excavated clonal fragments, and tiller density and biomass were studied from excavated turfs. Five life-cycle stages were determined: dormant buds. juvenile. mature, flowering and dead tillers. A stage-based transition matrix model was developed to estimate the long-term dynamics of the sedge populations and to compare life-history strategies between ungrazed and grazed populations. Short-term and retrospective models, based on the growth during the sampling year and during the lifetime of the clonal fragments. respectively, were compared to see how well the short-term model can describe demography of long-lived plants. According to the short-term model, tiller populations were decreasing (lambda < 1 except for C. membranacea), whereas the retrospective model indicated that the tiller populations were increasing, Tiller population growth rates did not differ between ungrazed and grazed habitats. Nevertheless, the similar growth rates may be obtained by balanced differences in the vital rates between plants of the two habitats. The plants in the ungrazed habitat tended to remain in their current life-cycle stage, whereas plants in the grazed habitat moved quickly to the next stage and died earlier. C. aquatilis ssp, stans appears to gain a competitive advantage over the other species under intensive grazing, as indicated by the higher tiller density and greater below-ground biomass in grazed vegetation. The greater amount of below-ground biomass apparently buffers C aquatilis ssp.stans against grazing better than the other species.